Method

In small saucepan, combine 1/2 cup white vinegar and 1/4 cup granulated sugar. Bring to boil, stirring until sugar has dissolved. Remove pan from heat and add 1 sprig fresh tarragon. Set aside. 

In saucepan of boiling water, blanch 1/4 cup mustard seeds for 15 seconds. Drain in fine sieve. Repeat 3 times using new water (this will reduce the bitterness of the mustard seeds). Drain seeds and place in 1-cup glass jar. Pour reserved marinade over seeds, seal jar and refrigerate for at least 12 hours. (Make-ahead: Can be refrigerated for up to 3 months. Drain mustard seeds before using.)

Makes about 1/2 cup.